1. Field of the Invention
The present invention relates to a picture reading method, program and device, a principal color extracting method, program and device, and an image dividing method, program and device, each being capable of reading pictures in an input image with high accuracy.
The present application claims priority of Japanese Patent Application No. 2004-126675 filed on Apr. 22, 2004, which is hereby incorporated by reference.
2. Description of the Related Art
Conventionally, various types of picture readers (picture reading devices) are proposed that automatically recognize pictures contained in an image picked up by an image pickup device, for example, a CCD (Charge-Coupled Device) camera or a like. As examples, a “number plate recognizing device” is disclosed in Japanese Patent Application Laid-open No. 2000-251016 and a “number reader (number reading device)” is disclosed in Japanese Patent Application Laid-open No. 2001-222680. These devices employ a method of recognizing a character being a picture by using a gray-scale image.
In Japanese Patent Application Laid-open No. 2003-85535, a “position recognizing method for a road guiding sign” is disclosed. In this method, a component of a color registered in advance is extracted from a color image of the road guiding sign and a character region is then extracted by the extraction of the component of the color to perform character recognizing processing.
Furthermore, an “image separating method and its device” is disclosed in Japanese Patent Application Laid-open No. Hei 07-73313. In the disclosed technology, an image region is over-separated according to a color and the over-separated regions are combined into character and background regions so that the character region is extracted in a stable manner without being affected by a shadow and/or a glare (highlight).
An image of a subject to be picked up by an image pickup device is multifarious. For example, there is a great difference in a displaying method between a number plate to display a registration number of a vehicle being employed in Japan (simply called a number plate) and a license plate to display a registration number of a vehicle being employed in the North America (simply called a license plate). The displaying method of the number plate is comparatively simple, however, the displaying method of the license plate is complicated when compared with the number plate. More specifically, in the case of the displaying method of the license plate, a design and/or format including a color to be used for its background are diversified and a font and its color to be used for a character are also various.
For example, FIG. 23 is an example of an image of a license plate in which a word “California” is written in a red color in its upper portion and characters “123” representing a license number are written in a bluish green color in its center portion. As shown in FIG. 23, there is a background portion, in which gradation processing is performed using a dark blue color in an upper half of the license plate, being changed gradually to be a white color toward a center portion from its upper portion and there is a background portion, in which gradation processing is performed using a red color in a lower half of the license plate, being changed gradually to be a white color toward a center from its lower portion.
FIG. 24 is a diagram showing an example of an image obtained by displaying the color image of FIG. 23 on a gray scale. In this case, since color gradation processing is performed in the character portion, in some cases, pixel values in the character regions are not always a maximum or a minimum in an input image.
As a result, the above conventional technologies have technical problems. That is, even when threshold processing on a pixel value to binarize the pixel is performed on an image represented on a gray scale by using the technology disclosed in the Japanese Patent Application Laid-open Nos. 2000-251016 and 2001-222680 and a character region is extracted using the above pixel, the character region such as “California” and/or “123” cannot be extracted with high accuracy.
Due to these technological problems, when the conventional method using the gray-scale image is used, the character region cannot be separated accurately from its background in the license plate and, therefore, accurate and automatic reading of a license number is difficult.
As one technological means to solve these problems, a method may be provided in which a character region is obtained by extracting a region having a color corresponding to a license number from a color license plate image acquired by picking up the license plate image using a CCD camera.
For example, as one of the technologies described above, by using the method disclosed in the Japanese Patent Application Laid-open No. 2003-855353, a method is available in which a component of a color registered in advance is extracted from a color image of a license plate and, by using the extracted component of a color, a character region is extracted and character recognizing processing is performed.
However, the technology disclosed in the Japanese Patent Application Laid-open No. 2003-855353 has a following problem. That is, when character recognizing processing is performed, by using the technology disclosed in the Japanese Patent Application Laid-open No. 2003-855353, on a license plate which employs very diversified colors, it is difficult to register, in advance, colors that may be used for the character. As a result, when the method for obtaining a character region by extracting colors registered in advance is used, the character region and background region can not be accurately separated from the license plate and automatic reading of a license number is difficult.
Furthermore, a factor that interferes with successful separation of a character region and background region is that, when the technology disclosed in the Japanese Patent Application Laid-open No. 2003-855353 is used outside, lighting conditions differ depending on environments in which an image is picked up during daytime, in evening, or at night and, therefore, even if an image of the same plate is picked up, color information is not uniform depending on the environment in which the image is picked up. For example, in the case where a license plate has a sky blue background and dark blue characters, when an image of the license plate is picked up in a state in which the license plate is lighted by the afternoon sun, a red component in an entire image becomes very intense and, therefore, when the image of the license is picked up during the daytime, even in the region in which a blue component becomes most intense, the red component becomes most intense in some cases.
Therefore, accurate extraction of a character region is difficult only by using the technology disclosed in the Japanese Patent Application Laid-open No. 2003-855353, which makes it difficult to automatically read a license number with high accuracy. That is, the above technology presents a problem in that, when a character region is extracted from a license plate, what color is used for a character in the character region is unknown and, therefore, it is difficult to extract a character region in a stable manner by using the method in which a component of a color having been registered in advance is extracted from a color image. As described above, in an outdoor environment, color components making up a character region are separated into a plurality of color components due to adverse effects caused by shadow areas or glare (highlight) areas on the license plate, which makes it more difficult to achieve stable extraction of a character region.
To solve such technological problems as described above, such technology disclosed in Japanese Patent Application Laid-open No. Hei 07-73313 is available. In the disclosed technology, by over-dividing a region of an image based on a color and combining the over-separated region, a character region can be extracted in a stable manner without being affected by shadow areas and/or glare (highlight) areas. However, in the technology disclosed in the Japanese Patent Application Laid-open No. Hei 07-73313, in the case of an image of a subject to be picked up which is made up of a color and shape having been defined in advance such as a road sign, it is possible to judge whether or not a color and shape is to be combined by comparing the color and shape with those contained in a dictionary stored in advance; however, when the above technology is applied to a license plate having a variety of colors and designs, it is necessary that an enormous amount of data about colors and shapes has to be stored in advance, which is not realistic.
Moreover, a region to be combined according to the technology disclosed in the Japanese Patent Application Laid-open No. Hei 07-73313 is not always a character region which is desired to be extracted. That is, there is a problem that, whether or not a character is extracted exactly is not judged substantially unless whether or not an extracted object is recognized as a character. Therefore, the above technology has a problem in that, since only a region extracted according to a reference which does not always coincide with a reference for character recognition defined in the above technology is extracted as a candidate for a character, accurate extraction of a character region is difficult. Therefore, a technological problem remains still unsolved in that accurate and automatic reading of a license number is difficult.